In a distributed wireless communication network including network nodes communicating via transit radio links, it is often desirable to manage and to coordinate the transit radio links. One of the problems associated with coordinating the transit links is to organize a scheme in which each transit radio link beam at a network node is aligned with neighbouring network nodes and operating at a correct frequency channel and polarization to accept traffic from or send traffic to the neighbouring nodes. Generally, there is only one transit link transceiver in any node, such that the transit link transceiver must be shared among the beams, channels and polarizations and coordinated with the neighbouring nodes to operate the transit links.
One known coordination technique is to synchronize all network nodes and provide each network node with a common clock and means of identifying time slots. The nodes then organize the time slots and beams to coordinate their communications on the transit links. However, a synchronized approach requires distribution of a reference time between network nodes and maintenance of synchronized clocks in each node, which tends to be difficult in packet-based communication networks, for example. In addition, allowances in time slots for clock jitter that often affects synchronized systems significantly reduce efficiency. Similarly, smaller time slots reduce waste if a packet does not fill a slot, but the timing accuracy and resolution required for small time slots is difficult to achieve with uncoordinated nodes. Small slots are thus preferred to reduce waste time, but are harder to make.
Further, with fixed length or synchronized slots, network nodes are not easily able to reallocate unused portions of slots to other traffic to adapt to packet flow. In packet-based system, packets are not always the same length, and can arrive at different times and potentially along different transit paths, so there is no steady flow over each transit link. To facilitate flow of packets, a time slot is normally allocated to each transit link, but its capacity is wasted if there is no traffic for that transit link. This capacity cannot be easily reallocated to other links that have excess traffic.
A known synchronous transit link scheme is the point control function (PCF) of the IEEE 802.11 standard. Those skilled in the art will appreciate that “802.11” refers to a set of specifications, available from the Institute of Electrical and Electronics Engineers (IEEE), relating to wireless local area networks. This scheme requires the synchronization of all the nodes subtending to a single access point, the point controller or PC. The PCF is also designed only for an environment with a single access point AP with a number of subtending stations (STA), and is not applicable to multi-link or multi-hop distributed networks. Further, the PCF uses a general polling technique to discover traffic, which is inefficient in that nodes must be polled even if they have no traffic to send. Traffic is also delayed until the node is polled. In this scheme, any communication between nodes that is not overheard by all subtending nodes may adversely affect synchronization.
Asynchronous transit links avoid some of the above shortcomings of synchronized systems. With asynchronous transit links, however, there is a possibility that a network node will not receive service due to high levels of traffic on other neighbouring nodes. This may block the node from sending its traffic and hence its subscribers may not receive satisfactory service. In addition, asynchronous links are not well suited to the passage of packets that require regular transmission times and low jitter, such as packets for speech or video services. Such packets are often described as having Quality of Service (QoS) parameters, including, for example, maximum delay and maximum jitter in packet transmission times.
In asynchronous networks, some network nodes may also be “lost” to the network if their transit radio links are affected by interference from external radio operations or physical blockage of the beams, for example.